The present invention relates to a mixer for a receiver for a portable radio communication apparatus, and in particular to a mixer for a direct conversion receiver.
A general trend in portable communication apparatus is the reduction in volume, weight and power consumption of such apparatus. This has led to efforts towards reducing the number of elements and devices necessary to perform the functions associated with portable communication devices. In particular, the radio frequency front end of portable apparatus, which typically comprises a number of down-converting stages, is an area in which a reduction in the number of elements and devices would be beneficial.
One approach to reduce the number of stages in the radio frequency front-end is to convert a received radio frequency carrier signal down to Van Intermediate Frequency (IF) in a single step. This is termed direct conversion and is carried out in receivers known by any one of the terms homodyne or zero IF receivers, as well as direct conversion receivers. To carry out direct conversion, a local oscillator signal (LO) having the same frequency as the radio frequency carrier signal (ie. the LO is "on-channel") is mixed with the carrier signal in a suitable non-linear device such as a mixer diode. The output of the mixer contains the sum and difference of the LO and the carrier signal. Thus a mixer product exists at twice the carrier signal, and also at DC (zero Hz). The high frequency mixer products can be filtered by a suitable low pass filter. Once the radio frequency carrier signal has been down-converted, the modulating signal may be de-modulated using an appropriate demodulator, e.g. an I/Q demodulator for an I/Q modulating signal.
In the field of radio telephony, particularly cellular telephony, use of a direct conversion receiver is not without certain drawbacks.
Radio telephones operating in the same cell of a cellular telephone network typically receive control signals on the same signal frequency as each other. This is necessary since such control signals are often so-called "broadcast" control signals such as paging signals which need to be received by all the telephones operating in the same cell. Signals received by radio telephones are of very low power, e.g. below -100 dBm and the sensitivity of the receivers is adapted accordingly.
Against the requirement for radio telephones in the same cell to be tuned to common frequencies, the introduction of direct conversion receivers with their strong on-board LO signals present the problem of interference. More specifically, spurious LO leakage from the receiver into the antenna can cause in-band interference with other nearby receivers tuned to the same channel. Therefore, such a receiver would require a very high level of isolation between the local oscillator and the antenna in order to avoid swamping or saturating receivers of any nearby radio telephones. Such high levels of isolation are very difficult to achieve due to stray capacitances directly coupling local oscillator energy into the antenna. This is the case even when conventional mixers such as a Gilbert cell is used.
Known methods addressing the problem of LO leakage include:
providing increased reverse-isolation in the RF path to the antenna, for example introducing multi-stage RF amplifiers, PA1 inserting an isolator, PA1 reducing the local oscillator drive power to very low levels.
However, these methods have shortcomings, in particular, they have been found to be detrimental to dynamic range and degrading to sensitivity, as well as being expensive to implement
Against this background, the present invention aims to address the previously described drawbacks.